Semiconductor memory having electrically erasable and programmable semiconductor memory cells

ABSTRACT

An electrically alterable non-volatile multi-level memory device and a method of operating such a device, which includes setting a status of at least one of the memory cells to one state selected from a plurality of states including at least first to fourth level states, in response to information to be stored in the one memory cell, and reading the status of the memory cell to determine whether the read out status corresponds to one of the first to fourth level states by utilizing a first reference level set between the second and third level states, a second reference level set between the first and second level states and a third reference level set between the third and fourth level states.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application. Ser.No. 11/870,196, filed Oct. 10, 2007, which, in turn is a DivisionalApplication of Ser. No. 11/206,995 (now U.S. Pat. No. 7,295,467), filedAug. 19, 2005, which, in turn is a Continuation of application Ser. No.11/043,114, filed Jan. 27, 2005 (now U.S. Pat. No. 7,002,847), which isa Continuation of application Ser. No. 10/872,515, filed Jun. 22, 2004(now U.S. Pat. No. 6,891,759), which is a Continuation of applicationSer. No. 10/685,441, filed Oct. 16, 2003 (now U.S. Pat. No. 6,771,542),which is a Continuation of application Ser. No. 10/304,046, filed Nov.26, 2002 (now U.S. Pat. No. 6,683,812, which is a Continuation ofapplication Ser. No. 09/944,406, filed Sep. 4, 2001 (now U.S. Pat. No.6,493,273); which is a Continuation of application Ser. No. 09/537,722,filed Mar. 30, 2000 (now U.S. Pat. No. 6,285,595); which is aContinuation of application Ser. No. 09/085,173, filed May 28, 1998 (nowU.S. Pat. No. 6,052,315), the entire disclosures of which are herebyincorporated by reference.

Keeping pace with the development of portable information devices,storage devices using a writable nonvolatile memory as a storage mediumare rapidly gaining popularity in recent years.

However, the cost-per-unit-capacity of a storage device using anonvolatile memory as a storage medium is higher than that of a storagedevice using a magnetic disk as a storage medium. Therefore, equipmentrequiring a large storage capacity often employs storage devices using amagnetic disk as a storage medium.

Under these circumstances, there has been a demand for an increasedstorage capacity in developing nonvolatile-memory-based storage devices.

Multilevel memory technology is a solution to meet this demand.

The multilevel memory technology involves a control over a potential ofa floating gate provided in an electrically erasable and programmablenonvolatile semiconductor memory cell so that the potential belongs toone of a plurality of predetermined potential levels.

This technology also identifies a potential stored in a sell by checkingwhich potential level such potential belongs to. Through theseoperations, a single cell is allowed to deal with multilevel data.

The aforementioned technology thus opens the way to the recording ofdata consisting of a plurality of bits in a cell unlike conventionaltechnologies that allow only one-bit data to be recorded in a cell. As aresult, large-capacity storage can be implemented.

In the multilevel memory technology, the operation of writing data to acell is performed with considerations given to provide a margin betweena desired potential level and a potential level adjacent to the desiredpotential level by controlling the setting of a potential to a floatinggate more finely.

With respect to the reading of data written in a cell, techniques aredisclosed in ISSCC95/Feb. 16, 1995/Digest of Technical Papers: Session 7“Flash Memory” TA 7-7 (pp. 132 to 133): A multilevel-Cell 32 Mb FlashMemory (INTEL Corporation), and JP-A-4-507320.

In the former technique, the potential level stored in a cell isidentified from a plurality of predetermined potential levels throughthe operation of discriminating the potential stored in the cell (theoperation of discriminating one of two levels) performed for a pluralityof times. As a result of these operations, data consisting of aplurality of bits written in the cell is determined.

Let us take an example in which two-bit data is written to a single cellby setting the potential to be stored in the cell to one of four levels.

In this example, the four levels are grouped into two. A discriminatingoperation is performed to determine which group the potential stored inthe cell belongs to.

Then, the group to which the potential stored in the cell belongsdetermined from the result of the discriminating operation is furtherdivided into two subgroups, and another discriminating operation isperformed to determine which subgroup the potential stored in the cellbelongs to.

As a result of these operations, the level to which the potential storedin the cell belongs is identified from the predetermined four levels.Thus, the two-bit data written to the cell is determined.

On the other hand, in the latter technique, the level to which thepotential stored in a cell belongs is identified from a plurality ofpredetermined levels using a plurality of discriminating means whosediscriminating thresholds are different. Through this technique, dataconsisting of a plurality of bits written to the cell can be determined.

Let us take an example in which two-bit data is written to a single cellby setting the potential to be stored in the cell to one of four levels.

In this example, means for discriminating the first level and the secondto fourth levels among the four levels are provided, and means fordiscriminating the first and second levels and the third and fourthlevels are provided, and further means for discriminating the first tothird levels and the fourth level are provided. By causing thesediscriminating means to perform their discriminating operations once,the level to which the potential stored in the cell belongs isidentified from the four levels.

Through these operations, the two-bit data written to the cell isdetermined.

By the way, the read operation involved in the aforementioned multilevelmemory technology addresses the following problems.

In the technique in which the level to which the potential stored in acell belongs is identified from a plurality of predetermined levelsthrough the potential discriminating operation performed for a pluralityof times, data consisting of a plurality of bits is determined throughthe plurality of discriminating operations, and thus the read operationtakes time.

The seriousness of this problem increases with increasing number of bitsconstituting the data to be stored in a single cell. Thus, this problemimpairs the high-speed reading performance that is one of the advantagesa storage device using a nonvolatile memory as a storage medium has overa storage device using a magnetic disk as a storage medium.

In the technique in which the level to which the potential stored In acell belongs is identified from a plurality of predetermined levelsusing a plurality of discriminating means whose discriminatingthresholds are different, a plurality of discriminating means must beprovided, and thus the area of the chip is disadvantageously increased.

The seriousness of this problem also increases with increasing number ofbits constituting the data to be stored in a single cell. That is, iftwo-bit data is to be stored in a single cell, three discriminatingmeans are required per cell, which means that, if three-bit data is tobe stored in a single cell, seven discriminating means are required percell.

Such disadvantage, which is the increased chip area brought about by theincreased number of peripheral circuits, does spoil the advantage, whichis the increased storage capacity per array area given by the increasednumber of bits per cell.

SUMMARY OF THE INVENTION

The present invention has been made in view of the aforementionedcircumstances. The object of the present invention is, therefore, toprovide a semiconductor memory and an information storage device bothcapable of achieving multilevel memory technology without impairing datareading performance nor increasing chip area.

To achieve the above object, the present invention is applied to asemiconductor memory having an electrically erasable and programmablenonvolatile semiconductor memory cell, and such semiconductor memoryincludes:

means for setting a potential to the cell, the potential correspondingto a level indicated by a bit data string obtained by arranging aplurality of pieces of bit data to be stored in the cell in apredetermined order; and

means for discriminating or comparing the potential set to the cell bythe potential setting means with a reference potential.

The discriminating means sequentially reads a plurality of pieces ofone-bit data constituting the bit data string arranged in thepredetermined order from a piece of one-bit data corresponding to astarting bit of the bit data string every time the discriminating meansperforms a discriminating operation with respect to the cell by settingthe reference potential to a level, the level corresponding to a numberof bits in the bit data string, a number of times of discriminatingoperations to be performed with respect to the cell and a result of thediscriminating operation already performed with respect to the cell.

The discriminating means performs the discriminating operation, e.g., inthe following procedure.

In a first discriminating operation with respect to the cell, thediscriminating means discriminates the potential set to the cell bysetting the reference potential to an intermediate level between apotential level corresponding to a minimum level possibly indicated bythe bit data string when a value set to the starting bit of the bit datastring is 1 and values set to other bits are unknown and a potentiallevel corresponding to a maximum level possibly indicated by the bitdata string when a value set to the starting bit of the bit data stringis 0 and values set to other bits are unknown.

As a result of this operation the starting bit data is read.

In a second discriminating operation with respect to the cell andonwards, the discriminating means discriminates the potential set to thecell by setting the reference potential to an intermediate level betweena potential level corresponding to a minimum level possibly indicated bythe bit data string when values set from the starting bit to aso-far-read bit of the bit data string are the respective read valuesand a value set to a next to-be-read bit is 1 and values set to otherbits are unknown and a potential level corresponding to a maximum levelpossibly indicated by the bit data string when values set from thestarting bit to the so-far-read bit of the bit data string are therespective read values and a value set to the next to-be-read bit is 0and values set to other bits are unknown.

As a result of these operations, the to-be-read bit data is read.

By repeating the aforementioned discriminating operations sequentially,the pieces of bit data from the second to the final bit of the bit datastring obtained by arranging the plurality of pieces of bit data in thepredetermined order are sequentially read.

According to the semiconductor memory of the present invention havingsuch structure, a plurality of pieces of one-bit data stored in a cellcan be read on a one-bit data basis every time the discriminating meansperforms the discriminating operation with respect to the cell.

Therefore, multilevel memory technology can be achieved withoutimpairing data reading performance nor increasing chip area due to anincreased number of discriminating means.

In the semiconductor memory of the present invention, data is recordedand reproduced on a data block basis, a data block consisting of aplurality of bits. Further, a plurality of cells are arranged in thesemiconductor memory, each cell corresponding to the plurality of bitsconstituting the data block.

The potential setting means sets a potential to each of the plurality ofcells, the potential corresponding to a level indicated by a bit datastring obtained by arranging as many data blocks as a number of bitscorresponding to the cell in a predetermined order.

A plurality of discriminating means are arranged, each discriminatingmeans corresponding to each of the plurality of cells The discriminatingmeans may read a to-be-read data block from the plurality of cells byperforming the operation of discriminating the potential of thecorresponding cell for a number of times corresponding to a bit numberfrom a first in the bit data string, the one-bit data constituting theto-be-read data block.

A file-based storage device usually records and reproduces data on afile basis, a file consisting of a plurality of sectors. That is, aplurality of sectors are recorded and read by a single access commandoperation. Further, the order in which the plurality of sectors areaccessed is usually fixed.

When the semiconductor memory of the present invention is applied to afile-based storage device of the aforementioned type, each cell storesone bit from each of a plurality of sectors (data blocks) in apredetermined order. The potential to be set to each cell belongs to alevel corresponding to a level indicated by a bit data string obtainedby arranging a plurality of pieces of one-bit data respectivelycorresponding to the plurality of sectors in the order in which theplurality of sectors is accessed.

As a result of this arrangement, the sectors stored in a plurality ofcells can be read in the order in which the sectors are accessed everytime the discriminating means performs the discriminating operation.

That is, a piece of data can be retrieved from a sector without havingto go through with all of the potential discriminating operation that isrequired to be performed for a plurality of times. Therefore, an accesstime similar to that required for a two-level memory can be achieved.

For example a four-level memory cell can store two bits. A total of 4096cells can store 1024 bytes, i.e., two sectors (one sector=512 bytes) interms of the storage capacity of an ordinary hard disk drive (HDD).

Thus, the semiconductor memory of the present invention has 4096 cells,and the first to 4096th pieces of bit data in each of the two sectorsare stored in the first to 4096th cells.

The potential of each cell is set to a level corresponding to a levelindicated by a bit data string obtained by arranging two pieces of bitdata to be stored in the cell in the order in which the two sectors areaccessed.

As a result of this arrangement, the first discriminating operationperformed by a discriminating means dedicated to each of the 4096 cellsallows the previously accessed one of the two pieces of sector data tobe retrieved, and the second discriminating operation allows the other,subsequently accessed one of the two pieces of sector data to beretrieved.

That is, the sector data can be read every time the discriminating meansperforms the discriminating operation, and thus an access time similarto that required for a two-level memory can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an architecture of a memory chipwhich is an embodiment of the present invention and to which amultilevel memory technology is applied;

FIG. 2 is a diagram explaining a potential level for storing pieces ofdata which have a bit number and which are stored in data block buffers5 and 6;

FIG. 3 is a schematic diagram showing a configuration of a read controlsection 44;

FIG. 4 is a diagram explaining a data discriminating process to beperformed when two pieces of bit data are read from the potential levelof a floating gate of a cell in a memory block 3 and the read pieces ofdata are stored in the data block buffers 5 and 6;

FIG. 5 is a diagram explaining a modification of the embodiment shown inFIG. 1 in which an information area for storing management informationis appended to each of memory blocks;

FIG. 6 is a diagram explaining how a discriminating operation isperformed using a value stored in a record storing area 34 shown in FIG.5; and

FIG. 7 is a schematic diagram showing a configuration of an informationstorage device using the memory chip shown in FIG. 1

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will now be described.

The following presents an example in which four levels (two-bit data)are stored per cell.

FIG. 1 is a schematic diagram showing an architecture of a memory chipwhich is an embodiment of the present invention and to which amultilevel memory technology is applied.

A memory chip 1, which is an embodiment of the present invention,comprises an electrically erasable and programmable read only memory(EEPROM) array 2, a data control circuit 4, a data block buffer 5A, adata block buffer 6B and an input/output (I/O) control circuit 7.

The EEPROM array 2 includes a plurality of electrically erasable andprogrammable nonvolatile semiconductor memory cells (hereinafterreferred to as the “cell” whenever applicable).

A memory block 3 is a set of cells and provides a unit for erasing datafrom the EEPROM array 2.

In order to deal with data on the basis of a sector (one sector=512bytes=4096 bits), which is a storage capacity unit generally applied tomagnetic disk devices, a single memory block is designed to have 4096memory cells in this embodiment.

As described above, this embodiment is designed to store four levels(two-bit data) per cell, which means that a single memory block canstore 8192 bits.

The data block buffer A5 and the data block buffer B6 temporarily storedata to be written to or read from the EEPROM array 2. Each buffer isdesigned to store 4096 bits in this embodiment.

The I/O control circuit 7 connects the memory chip 1 to a system bus ofa storage device on which the chip 1 is mounted. The circuit 7 controlsinput and output of data by receiving addresses, command codes orcontrol signals.

The memory chip 1 receives writing data from an external source on asector basis. The I/O control circuit 7 stores the received data in thedata block buffer A5 or B6. The circuit 7 selects a buffer to which thereceived data is written based on a sector address and on the writtenstate of a memory block 3 specified by the sector address.

The I/O control circuit 7 further sends data read from a memory block 3and stored in the data block buffer A5 or B6 to an external destination.

As shown in FIG. 1, the data control circuit 4 has a write controlsection 42 and a read control section 44.

The write control section 42 writes data stored in the datablock-buffers A5 and B6, respectively, to a corresponding memory block 3of the EEPROM array 2.

Write operation is performed in the following procedure.

First, 4096-bit data stored in each of the data block buffers A5 and B6is retrieved from each of the buffers A5 and B6, and the 4096 bitsconstituting such data are numbered on a bit basis. More specifically,the 4096 bits of data stored in each buffer A5 or B6 is sequentiallynumbered from the first bit to the final 4096th bit.

Then, a level indicated by a bit data string is calculated. The bit datastring is obtained by arranging the data whose bits are numbered on abit basis in the order of the data block buffer A5 and the data blockbuffer B6. Since the bit data string consists of two bits, four levelscan possibly be indicated by the bit data string.

Then, each of the first to 4096th cells of a memory block 3 iselectrically charged so that each cell is set to a potentialcorresponding to a level indicated by the bit data string whose bits aregiven a bit number corresponding to the cell.

The write operation will be described in more detail.

FIG. 2 is a diagram explaining a potential for writing pieces of datawhich have a bit number and which are stored in the data block buffersA5 and B6, respectively.

FIG. 2 presents how four levels each specifying a potential aredetermined from a piece of data (two levels) having a bit number storedin the data block buffer A5 and a piece of data (two levels) having thesame bit number stored in the data block buffer B6.

In FIG. 2, a portion 11 schematically depicts distributions of potentiallevels that can be set to the floating gate of a cell in the EEPROMarray 2. The potential to be set to a cell is one of four levelscorresponding to four possible values given by data to be written to thecell.

For example, if a piece of data in the data block buffer A5 is “1” and apiece of data in the data block buffer B6′is “0” with respect to a bitnumber, the level indicated by the bit data string is 2 when thesepieces of data are arranged in the order of the data block buffer A5 andthe data block buffer B6.

In this case, the potential to be set to a cell corresponding to the bitnumber is within the second potential range from the top in portion 11of FIG. 2.

If, e.g., a piece of data in the data block buffer A5 is “0” and a pieceof data in the data block buffer B6 is “1” with respect to a bit number,the level indicated by the bit data string is 1 when these pieces ofdata are arranged in the order of the data block buffer A5 and the datablock buffer B6;

In this case, the potential to be set to a cell corresponding to the bitnumber is within the third potential range from the top in portion 11 ofFIG. 2.

If only the data block buffer A5 has data and the data block buffer B6has no data, the potential level is determined, assuming that data “1”is stored in each bit of the buffer B6. Therefore, the level indicatedby the bit data string is either “3” or “1.”

The potential setting procedure in this case is as follows. Upon storageof a piece of data in the data block buffer B6, such piece of data isretrieved from the buffer B6, and the retrieved piece of data and acorresponding piece of data already written to the data block buffer A5are referred to, so that the potential is set.

For example, when a piece of data is stored in the data block buffer B6,the read control section 44, which will be described later, reads apiece of data already written to the data block buffer A5 to obtain thevalue of the data read from the buffer A5.

Then, the level indicated by the bit data string obtained by arrangingpieces of data in the data block buffers A5 and B6 in the order of thedata block buffer A5 and the data block buffer B6 is calculated for eachbit number.

Successively, each of the cells in the memory block 3 is charged againto a potential level corresponding to the level indicated by the bitdata string whose bit number corresponds to the cell.

Further, the values of data stored in the data block buffer B6 arechecked for each bit number, for example.

It should be reminded here that for those cells corresponding to the bitnumbers in which data “1” is stored in the data block buffer B6, thepotential levels are set, during the operations of writing data storedin the data block buffer A5, by assuming that each bit of the data blockbuffer B6 contains data “1.” For this reason, write operations areskipped for those cells.

On the other hand, the potential levels are set to those cellscorresponding to the bit numbers in which data “0” is stored in the datablock buffer B6 in such a manner that the potential levels are decreasedby one order of magnitude (i.e., by a potential level necessary to movedown to the next potential range as seen in portion 11 of FIG. 2).

To summarize, the potential control is effected as follows. Thepotential to be set to a cell corresponding to a bit number is equal toa level indicated by a bit data string that is obtained by arranging apiece of data having the bit number in the data block buffer A5 and apiece of data having the same bit number in the data block buffer B6 inthe order of the data block buffer A5 and the data block buffer B6.

These steps are taken in consideration of the following characteristicsof the EEPROM.

The EEPROM is in the highest potential state when erased, i.e., when theEEPROM is ready to be written with data of any level indicated by thebit data string. This “erase” state corresponds to level “3” indicatedby the bit data string.

On the other hand, the lowest level corresponds to level “0” indicatedby the bit data string. The lowest level is brought about by chargingelectrons to the floating gate of a cell.

Further, the once decreased potential levels can be increased by erasingdata collectively on a data block 3 basis.

It should be emphasized that the potential level can be decreased on acell basis but that the potential level is increased on a data blockbasis through erase operation.

In other words, the EEPROM allows data to be written additionally in asmall storage capacity unit by, e.g., overwriting. However, once thepotential level is changed to a level from the erase state, the EEPROMdoes not allow potential levels higher than that level to be set unlessdata are erased on a data block basis.

To reduce this burden borne by the user, this embodiment is designedsuch that, when only the data block buffer A5 has data, each of the bitsin the data block buffer B6 is assumed to contain data “1” so thateither level “3” or “1” is indicated by the bit data string. That is,considerations are given to set the potential to a higher level in thisembodiment.

Further, when a piece of data is thereafter written to the bit in thedata block buffer B6 and the potential indicated by the bit data stringis changed to level “2” or “0” as a result of such data writing, thisembodiment allows a potential corresponding to the changed level “2” or“0” to be set to the cell corresponding to such bit.

Therefore, once a piece of data stored in the data block buffer A5 hasbeen written. such piece of data stored in the buffer A5 cannot berewritten unless data are erased on a data block basis.

However, it is no longer necessary to write a piece of data stored inthe data block buffer B6 together with a corresponding piece of datastored in the data block buffer A5.

If data is stored only in the data block buffer B6 and no data is yetstored in the data block buffer A5, the potential level can bedetermined by assuming that each bit in the buffer A5 contains data “1.”In this case, the level indicated by the bit data string corresponds to“3” or “2.”

The potential setting procedure in this case is as follows. Upon storageof a piece of data in the data block buffer A5, such piece of data isretrieved from the buffer A5, and the retrieved piece of data and acorresponding piece of data already written to the data block buffer B6are referred to, so that the potential is set.

These steps are also taken in consideration of the aforementionedcharacteristics of the EEPROM.

The potential of a cell can be set to a desired level by graduallyadding up charges.

Further, the potential of a cell may also be set to a desired level byfirst storing charges to a level one order of magnitude lower than thedesired level at a stretch and then gradually adding up charges to thedesired level. This technique permits quick data writing to the cell.

The read control section 44 controls the reading of data from a memoryblock 3 of the EEPROM array 2.

FIG. 3 is a schematic diagram showing a configuration of the readcontrol section 44.

As shown in FIG. 3, the read control section 44comprises adiscriminating circuit 441, a timing control circuit 442, a referencepotential control circuit 443 and a buffer control circuit 444.

The discriminating circuit 441 has discriminators 445 ₁ to 445 ₄₀₉₆ thatrespectively correspond to the cells in each memory block 3. Thediscriminating circuit 441 sequentially discriminates one data blockfrom two data blocks stored in the memory block 3 using thediscriminators 445 ₁ to 445 ₄₀₉₆.

The reference potential control circuit 443 sets a reference potential(discriminating threshold) of each of the discriminators 445 ₁ to 445₄₀₉₆ arranged in the discriminating circuit 441.

The buffer control circuit 444 controls the data. block buffers A5 andB6 to specify the destination for storing the data block discriminatedby the discriminating circuit 441.

The timing control circuit 442 controls the operation timings of thevarious parts of the read control section 44.

The thus configured read control circuit 44 reads data in the followingprocedure.

Potential levels to be set to the cells in a to-be-read memory block 3are fed to the corresponding discriminators 445 ₁ to 445 ₄₀₉₆,respectively.

Concurrently with this operation, the reference potential controlcircuit 443 sets a reference potential to each of the discriminators 445₁ to 445 ₄₀₉₆. That is, the reference potential to be set is anintermediate level between a potential level corresponding to a minimumlevel that can be indicated by a two-bit data string when the value setto the first bit of the two-bit data string is “1” and that set to thesecond bit is unknown and a potential level corresponding to a maximumlevel that can be indicated by the two-bit data string when the valueset to the first bit is “0” and that set to the second bit is unknown.

As a result, the discriminating circuit 441 reads the data block storedin the data block buffer A5 from the two data blocks stored in thememory block 3 (the two data blocks being the data block stored in thebuffer A5 and the data block stored in the buffer B6).

The buffer control circuit 444 controls the data block read from thememory block 3 so that the read data block is stored in the data blockbuffer A5.

Then, the reference potential control circuit 443 sets a referencepotential for each of the discriminators 445 ₁, to 445 ₄₀₉₆. That is,the reference potential to be set to a discriminator is an intermediatelevel between a potential level corresponding to a minimum level thatcan be indicated by a two-bit data string when the value set to thefirst bit of the two-bit data string is the value discriminated by thediscriminator through the aforementioned operation and that set to thesecond bit is “1” and a potential level corresponding to a maximum levelthat can be indicated by the two-bit data string when the value set tothe first bit is the value discriminated by the discriminator throughthe aforementioned operation and that set to the second bit is “0.”

As a result, the discriminating circuit 441 reads the data block storedin the data block buffer B6 from the two data blocks stored in thememory block 3 (the data block stored in the buffer A5 and the datablock stored in the buffer B6).

The buffer control circuit 444 controls the data block read from thememory block 3 so that the read data block is stored in the data blockbuffer B6.

The two data blocks stored in the memory block 3 are read in the orderof the data block buffer A5 and the data block buffer B6 in this way.

The read operation will be described in more detail.

FIG. 4 is a diagram explaining a data discriminating process to beperformed when two pieces of one-bit data are read from the potentiallevel of the floating gate of a cell in a memory block 3 and the readpieces of data are stored in the data block buffers A5 and B6,respectively.

In a manner similar to the case shown in FIG. 2, the potential of thefloating gate is set to and held at one of the four distributions in thecell.

The thus set and held potential is discriminated using a discriminatorcorresponding to the cell in such a manner that the potential belongs toeither one of two groups: a group having levels “3” and “2,” and a grouphaving levels “1” and “0.” If the set and held potential belongs to theformer group, “1” is stored in the data block buffer A5 having a bitnumber corresponding to the cell. If the set and held potential belongsto the latter group, “0” is stored in the buffer A5 having the bitnumber corresponding to the cell.

That is, data to be stored in the data block buffer A5 can be readthrough only one discriminating operation.

Further, data to be stored in the data block buffer B6 can be readthrough another discriminating operation.

For example, if it is found out through the first discriminatingoperation that the set and held potential belongs to level “3” or “2,”another discriminating operation is performed to find out which levelthe potential is actually set to, either level “3” or “2.” If thepotential is found out to be set to level “3,” “1” is stored to the datablock buffer B6 having a bit number corresponding to the cell, whereasif the potential is found out to be set to level “2,” “0” is stored inthe buffer B6 having the bit number corresponding to the cell.

Further, if it is found out through the first discriminating operationthat the set and held potential belongs to level “1” or “0,” anotherdiscriminating operation is performed to find out which level thepotential is actually set to, either level “1” or “0.” If the potentialis found out to be set to level “1,” “1” is stored to the data blockbuffer B6 having a bit number corresponding to the cell, whereas if thepotential is found out to be set to level “0,” “0” is stored in thebuffer B6 having the bit number corresponding to the cell.

As described above, this embodiment allows two data blocks stored in amemory block 3 can be sequentially read every time the discriminatingcircuit 441 performs the discriminating operation.

Therefore, multilevel memory technology can be achieved withoutimpairing data reading performance not Increasing chip area due to theincreased number of discriminators.

If the two data blocks stored in a single memory block 3 are two-sectordata belonging to a single file, the two-sector data are read from thememory block 3 in the same order as they are written.

In this case, it may be so arranged at the time the two-sector data arewritten to the memory block 3. that the I/O control circuit 7 stores thepreceding portion of the two-sector data in the data block buffer A5 andthe succeeding portion in the data block buffer B6.

Such arrangement allows the two-sector data written in the memory block3 to be read in the correct order.

On the other hand, the two data blocks stored in a single memory block 3are two-sector data respectively belonging to different files, thetwo-sector data may be accessed totally independently in some cases.

In such cases, if one portion of the two-sector data temporarily storedin the data block buffer A5 is requested to be read at the time thetwo-sector data are written to the memory block 3, such requestedportion of the two-sector data can be read by causing the discriminatingcircuit 441 to perform the discriminating operation once.

If, on the other hand, the other portion of the two-sector datatemporarily stored in the data block buffer B6 is requested to be readat the time the two-sector data are written to the memory block 3, suchrequested portion of the two-sector data can be read by causing thediscriminating circuit 441 to perform the discriminating operationtwice.

However, if no data is yet written to the memory block 3 from the datablock buffer A5, the potential of the corresponding cell in the memoryblock 3 is set to a level corresponding to level “3” or “2” as describedabove. Therefore, the requested portion of the two-sector datatemporarily stored in the buffer B6 can be read through only onediscriminating operation.

Hence, not only data in the data block buffer A5 can be read from thememory block 3 if no data is written to the memory block from the datablock buffer B6, but also data in the data block buffer B6 can be readfrom the memory block 3 if no data is written to the memory block 3 fromthe data block buffer A5.

The example in which data is written to a cell by reducing the potentiallevel of the cell while charging electrons to the floating gate of thecell that is in the complete “erase” state has been described in thefirst embodiment. However, the present invention is not limited to thisexample.

It should be reminded that the potential level of a cell, whether it ishigh or low, in the erase state or in the write state differs from onememory to another. For example, some define “erase” as chargingelectrons to the floating gate of a cell that is in the complete writestate, and others define the “erase” state as setting the potentiallevel to “0” and the “write” state as setting the potential level to “1”or more. And such variations in the definition of the “erase” and“write” states, the present invention can take care of any of themecases by altering the set potential to an appropriate level.

Further, while the example in which the storage capacity of each memoryblock 3 in the EEPROM array 2 is 1024 bytes (=8192 bits) has beendescribed in the aforementioned embodiment, the present invention is notlimited to this example.

Still further, it is beneficial in the aforementioned embodiment toappend to each memory block 3 an information area that stores managementinformation of that block in order to efficiently manage stored data.

FIG. 5 is a diagram explaining a modification of the first embodiment,in which an information area is appended to each memory block in orderto store management information of that block.

Memory blocks 3 ₁ to 3 ₄, which are exemplary modifications of thememory blocks 3 shown in FIG. 1, have information areas for storingmanagement information.

Each of the memory blocks 3 ₁ to 3 ₄ comprises a data block storing area41 and a management information storing area 33. The data block storingarea 41 stores two data blocks (a data block temporarily stored in thedata block buffer A5 and a data block stored in the data block buffer B6shown in FIG. 1). The management information storing area 33 storesmanagement information of the data block storing area 41 such as storinglogic addresses, rewriting time information, stored data identificationinformation, and error detecting and correcting codes.

The example shown in FIG. 5 presents the data block storing area 41 ofthe memory block 3 ₁ with nothing written therein, the data blockstoring area 41 of the memory block 3 ₂ with a data block of the datablock buffer A5 written therein, the data block storing area 41 of thememory block 3 ₃ with a data block of the data block buffer B6 writtentherein, and the data block storing area 41 of the memory block 3 ₄ withthe data blocks of the data block buffers A5 and B6 written therein.

The management information storing area 33 includes a record informationstoring area 34 that stores storing record information as part of themanagement information.

In this example, it is designed to write “0” in the record informationstoring area 34 if nothing is stored in the data block storing area 41,“1” in the record information storing area 34 if the data block of thedata block buffer A5 is written in the data block storing area 41, “2”in the record information storing area 34 if the data block of the datablock buffer B6 is written in the data block storing area 41, and “3” inthe record information storing area 34 if the data blocks of the datablock buffers A5 and B6 are written in the data block storing area 41.

By the way, if “1” is written in the record information storing area 34,it means that the potential of each cell constituting the data blockstoring area 41 is set to a level corresponding to level “3” or “2,” orlevel “1” or “0” shown in FIG. 2.

Further, if “2” is written in the record information storing area 34, itmeans that the potential of each cell constituting the data blockstoring area 41 is set to a level corresponding to level “3” or “2”shown in FIG. 2.

Still further, if “3” is written in the record information storing area34, it means that the potential of each cell constituting the data blockstoring area 41 can be set to any level corresponding to level “3,” “2,”“1” or “0” shown in FIG. 2.

Therefore, by checking the value stored in the record informationstoring area 34, the operator can grasp the potential level that is setto each cell constituting a data block storing area 41. Hence, theoperator can determine how data should be read.

FIG. 6 is a diagram showing an exemplary application of the recordinformation storing area 34 and explaining the flow of steps to be takenin executing a discriminating process using a value stored In the recordinformation storing area 34.

The following describes a case where “2” is stored in the recordinformation storing area 34.

As described above, when “2” is stored in the record information storingarea 34, the potential to be possibly set to each cell constituting thedata block storing area 41 is level “3” or “2” (the levels indicated bythe solid lines in FIG. 6).

Therefore, the discriminating operation is required to be performed onlyfor those distributions indicated by the solid lines. That is, only onediscriminating operation is required to be performed.

In order to switch the discriminating operation based on a value storedin a record information storing area as described above using the memorychip according to this embodiment, the following circuit configurationmay be employed.

The discriminating circuit 441 is provided with means for discriminatingthe potential of a cell constituting the record information storing area34 before discriminating the potential of each cell constituting thedata block storing area 41.

In order to keep the reference potential constant at the time ofdiscriminating operation, it should be so designed that the cellconstituting a record information storing area 34 can store one bit (twolevels). Therefore, if four levels are to be stored in a recordinformation storing area 34 as described above, a record informationstoring area 34 consists of two cells.

In setting the reference potential, the reference potential controlcircuit 443 is caused to take into account the result of thediscriminating operation performed to the record information storingarea 34 by the discriminating circuit 441 before the discriminatingoperation is performed to the corresponding data block storing area 41.

That is when “2” is stored in a record information storing area 34, thereference potential is set to an intermediate level between levels “3”and “2” shown in FIG. 6.

Further, when “1” is stored in a record information storing area 34, thereference potential is set to an intermediate level between levels “2”and “1” shown in FIG. 6.

When “3” is stored in a record information storing area 34, thereference potential is set according to the procedure described withreference to the first embodiment of the present invention.

Record information can be written by causing the write control section42 to check the source of a data block, i.e., to check which data blockbuffer such data block is sent from and to refer to the recordinformation of the destination of such data block, i.e., to refer to therecord information of a memory block to which such data block is to bewritten when such data block is written to a data block storing area 41,

The reference potential at the time of discriminating operation may beswitched on the basis of an externally applied identifier such as acommand code instead of arranging an area for storing record informationin a memory block.

While the examples in which four levels (two bits) are stored in eachcell have been described in the aforementioned embodiments. the presentinvention is not limited to these examples. Also acceptable are examplesin which eight levels (three bits). sixteen levels (four bits) or levelsgreater than these are stored in each cell.

Before concluding the specification, an information storage device usingthe memory chip according to the aforementioned embodiments will bedescribed.

FIG. 7 is a schematic diagram showing a configuration of an informationstorage device using the memory chip shown in FIG. 1.

The information storage device comprises an interface 91 forcommunicating with a host computer and a memory chip controller 92 forcontrolling a plurality of memory chips 1. The interface 91 and thememory chip controller 92 may be those commonly used for conventionalinformation storage devices.

As described in the foregoing, according to the embodiments of thepresent invention, a plurality of pieces of one-bit data stored in acell can be read on a one-bit data basis every time the discriminatingmeans dedicated to the cell performs a discriminating operation.

Therefore, the present invention can achieve multilevel memorytechnology without impairing data reading performance nor increasingchip area due to an increased number of discriminating means.

1. A nonvolatile memory apparatus comprising: a nonvolatile memory; anda control unit for controlling writing data and reading data to or fromthe nonvolatile memory, wherein the nonvolatile memory includes pluralmemory cells, each of which is for storing plural bits of data thereinby shifting a threshold voltage thereof to one of plural thresholdvoltage ranges, wherein, in performing a first page writing, thethreshold voltage of a first memory cell of said plural memory cellsremains in a first threshold voltage range or shifts into a secondthreshold voltage range in accordance with data to be written into thefirst memory cell, wherein, in performing a second page writing, thethreshold voltage of the first memory cell remains in the firstthreshold voltage or the second threshold voltage, or shifts into athird threshold voltage range from the first threshold voltage range orinto a fourth threshold voltage range from the second threshold voltagerange, and wherein a second memory cell of said plural memory cells isconfigured to be used for storing information indicating whether thefirst page writing or the second page writing is performed on the firstmemory cell.
 2. A nonvolatile memory apparatus according to claim 1,wherein the nonvolatile memory comprises an amplifier for reading datastored in the first memory cell, and wherein, in performing a datareading, the amplifier receives a reference voltage, a voltage level ofwhich is determined in accordance with the information stored in thesecond memory cell.
 3. A nonvolatile memory apparatus according to claim2, wherein the second memory cell is included in third memory cells,which are configured to be used for storing management information ofthe first memory cell.
 4. A nonvolatile memory apparatus according toclaim 3, wherein one of the threshold voltage ranges indicates an erasestate, and the others indicate program states.